This invention relates to network analyzers employed for electrical circuit measurements and, more particularly, to calibration techniques for network analyzers. Specifically, the invention provides a novel configuration for a transmission line element employed in calibrating a network analyzer used for measurements in the microwave and millimeter-wave range, and a novel method for using the transmission line element in calibrating the analyzer.
Network analyzers are universally used for determining response characteristics of various devices under test, such as filter circuits (or more complex electrical circuits), in order to characterize the device or test it to assure that it meets specifications. The devices under test are either one-port or two-port circuits The accuracy of a network analyzer depends not only upon the design of the analyzer, but also upon calibration of the analyzer.
Accordingly, various one-port and two-port techniques have been developed for calibrating network analyzers. Known calibration techniques vary in complexity and accuracy.
Typically, these calibration techniques have involved the use of open-, short-, and load-circuit electrical measurements In traditional one-port calibration techniques, the test port of the network analyzer is open-circuited, and a measurement is taken. This process is repeated with the test port short-circuited, and finally with an impedance-matched load connected across the test port. These measurements are then utilized to calibrate the network analyzer. In traditional two-port calibration techniques, these same measurements are taken for each test port of the network analyzer individually, and, additionally, another measurement is taken by connecting the two test ports together to measure the transmission. Often, these techniques employ sliding loads and/or transmission lines that have lengths that are long relative to the wavelengths at the measurement frequencies of interest.
When a length of transmission line is used in connection with the calibration technique, the electrical measurement depends upon the mechanical accuracy of the transmission line. The challenge is therefore how to construct a transmission line having a fixed characteristic impedance when inserted into a network analyzer measurement configuration between the test port or ports of the analyzer, on the one hand, and a load or short, on the other hand This characteristic impedance should be accurately known to many decimal places.
However, problems have arisen with the connection of the inner conductor of known transmission lines. Heretofore, contact between the inner conductors of the transmission line and the network analyzer test port, or load or short, has been effected by collets or a pin-in-socket connection.
One known transmission line, available from various manufacturers, comprises an inner conductor rod having recesses formed in the ends. A collet, such as an APC-7 (trademark of Amphenol Corporation) snowflake, is inserted into the recess at each end of the conductor rod. The inner conductor is supported by insulators typically disposed near the ends of the conductor rod in the interstitial space between the rod and the barrel of the surrounding coaxial outer conductor. Unfortunately, the insulators adversely affect performance. Also, connections are not highly repeatable due to the variable compliance of the collets when the conductor rod is interfaced with similar collets associated with the mating inner conductors of the network analyzer test port and load or short.
Another known transmission line, for example, included in the Model Number HP 11637A calibration kit, available from Hewlett-Packard Company of Palo Alto, Calif., comprises an inner conductor rod which is not supported by insulators. The rod is shorter than the outer conductor, and also shorter than the distance to be spanned by the inner conductor A pin is formed at each end of the rod and extends outwardly away from the rod end into contact with a socket provided in the mating inner conductor of the network analyzer test port or load or short, as the case may be, opposite the rod end. The outer conductor surrounds the inner conductor and is typically provided with a threaded collar for engagement with a threaded sleeve extending from the test port or load or short. Unfortunately, the rod can be radially offset from the centerline of the structure to which the transmission line is connected, the conductor rod can be longitudinally shifted with respect to the outer conductor, or the inner conductor can bow.
An improved pin-in-socket connection is found in the Model Number 2653, available from Maury Microwave Corporation of Cucamonga, Calif., in which contact between the inner conductors is effected by a spring mechanism. The ends of the rod are provided with recesses. A spring-loaded pin resides in the recess at each end of the rod and is biased outwardly away from the rod end into contact with a socket provided in the mating inner conductor of the network analyzer test port or load or short opposite the rod end. Additionally, the inner conductor rod has conical ends, and countersinks are provided in the mating inner conductors of the network analyzer test port, load, or short to aid in aligning the inner conductors, but axial shift and bowing problems persist.
These misalignment problems and asymmetries result in irreproducibility of connections required for accurate network analyzer calibration. Therefore, it is desirable to provide a more accurate and repeatable connection between the inner conductors.